The present invention relates to a paper/board web coating apparatus, which is arranged to extend in its longitudinal direction in the transverse direction of the web to be coated, and which comprises a nozzle unit having at least one feeding chamber extending in the longitudinal direction of the coating apparatus, into which chamber is conveyed coating color by feeding means, and a nozzle slot in flow communication with the said feeding chamber, the said slot also extending in the longitudinal direction of the coating apparatus, and to which slot the coating color is supplied from the feeding chamber over the total longitudinal distance of the nozzle slot and further conveyed out of the outlet opening of the nozzle slot.
The aim of the present invention is to provide an improvement to a curtain coater intended for spreading the coating color of a paper/board web.
The use of a curtain coater for coating paper or board webs is increasing, because it exerts much less force on the web than blade coating/rod coating and, consequently, causes fewer interruptions resulting from the breaking of the web being coated, and thus improves runnability. Curtain coating does not give the same smoothness as blade coating, but the coverage attained is better than that attained with blade coating.
Curtain coaters can be divided into slot-fed and slide-fed coaters. In a slide-fed curtain coater, the coating color is supplied by means of a nozzle unit onto an inclined plane, along which the coating color flows towards the edge of the plane, whereby a curtain is formed as the coating color drips from the edge of the plane.
In slot-fed applicator beams, the coating color is pumped via a manifold into a narrow vertical slot on the lip of which the curtain is formed and drips onto the web. The coating color may be spread in one or more layers.
The curtain thus formed is guided by means of an edge guide, which is located on the edge of the feeding slot/feeding lip, as indicated by its name.
One problem with current coating color curtain coaters is the limited size of the coating color feed channels and, partly due to this, the difficulty of controlling the size of the flow slot. Controlling the transverse profile of the coating color spread on the web to be coated in different situations is problematic also in a prior art curtain coater. Normally, there are no means for controlling the profile. When designing a coating beam, the shape of the feed channels is determined, by means of which the overall profile may be affected. When the properties and/or feeding volume of coating color change, these have a distinct effect on the transverse profile which cannot be corrected. All imprecisions of manufacture also affect the profile irreparably.